How do I read a user-specified file in an emscripten compiled library?

Question

I\'m currently working on a file parsing library in C with emscripten compile support. It takes a file path from the user where it reads the binary file and parses it.

Answer 1

If you can provide a file via a form upload from your webpage, then on receiving the file upload "data" you can do something like this.

Refer here on how to achieve this

Sample code for your problem could be:

// Assuming data contains the base64 encoded fileData,
// you want the file to be present at "." location with name as myFileName
Module['FS_createDataFile'](".", myFileName, atob(data), true, true);

The detailed explanation of API used can be found here

This solution works for browsers where you do not have direct access to file system

Answer 2

You can also use a synchronous virtual XHR backed file. Beware that it requires that you execute your code in a web worker. If you want to support multiple files, you can access them using a virtual file system for an archive. The major plus of this approach is that it will defer loading until the file is actually read.

Answer 3

If you want compile this file directly into library you can use --preload-file or --embed-file option. Like this:

emcc main.cpp -o main.html --preload-file /tmp/my@/home/caiiiycuk/test.file

After that in C you can open this file normally:

fopen("/home/caiiiycuk/test.file", "rb")

Or you can use emscripten javascript fs-api, for example with ajax:

$.ajax({
    url: "/dataurl",
    type: 'GET',
    beforeSend: function (xhr) {
        xhr.overrideMimeType("text/plain; charset=x-user-defined");
    },
    success: function( data ) {
        Module['FS_createDataFile']("/tmp", "test.file", data, true, true);
    }
});

After that you can open this file from C. Also it is not best way to pass data into C code, you can pass data directly in memory, read about this.

Answer 4

Emscripten, as of today (June 2016), supports a filesystem called NODEFS which provides access to the local file-system when running on nodejs.

You need to manually mount the NODEFS filesystem into the root filesystem. For example:

EM_ASM(
  FS.mkdir('/working');
  FS.mount(NODEFS, { root: '.' }, '/working');
);

You can then access ./abc from the local filesystem via the virtual path /working/abc.